Distillation is employed in purification systems for removing or extracting contaminants from liquids by a process of evaporation. The simplest example of distillation is one in which the entire process is carried out by heating and cooling the liquid, wherein the liquid to be purified is introduced into a boiling chamber where the liquid's temperature is raised to the boiling point of the contaminant to be extracted. Subsequent addition of heat no longer raises the temperature of the contaminated liquid but serves only to vaporize the contaminate to be extracted. The vapor produced in the boiling chamber is then conveyed into a condensing chamber where it is cooled until it condenses and becomes pure liquid called "condensate".
To improve the energy efficiency of the distillation process, vapor compression is employed to recycle the heat used to vaporize the liquid. In vapor compression distillation, the vapor produced in the boiler is compressed before it is transferred to the condenser, whereby the condenser is maintained at a higher temperature than the boiler, and the heat released by the vapor as it condenses can be driven back into the boiler to produce more vapor.
Initially, most vapor compression distillation systems employed shell-in-tube type heat exchangers for their boiler/condenser assemblies; however, the construction and arrangement of the tubes presented an inadequate amount of surface area for heat transfer at low differential temperatures, and at higher differential temperatures the efficiency of the vapor compression distillation process was greatly reduced.
To overcome the disadvantages experienced with shell-in-tube type heat exchangers, a plate-in-frame type heat exchanger has been proposed to provide a boiler/condenser assembly, as disclosed in U.S. Pat. No. 4,671,856, wherein the boiling and condensing stages of the vapor compression distillation process takes place by conveying the feed material into a series of alternating boiling and condensing chambers on opposite sides of vertically extending common plate members. The feed material is then caused to boil evenly over substantially the entire boiling surface of the common plate member. The heat required to boil the feed material in the boiling chamber is almost entirely provided by the vapor condensing at a higher temperature in the adjacent condensing chamber. As compressed vapor condenses in the condensing chamber, on the opposite side of the common plate member, its heat of vaporization is directed back into the boiling chamber, through the common plate member under the influence of the differential temperature which exists between the adjacent boiling and condensing chambers.
The boiler/condenser described in U.S. Pat. No. 4,671,856 employs flat plates for the vertically extending common plate members, whereby the vapor condenses on the plate in a thin film which flows by gravity vertically downwardly on the surface of the plate, resulting in a film thickness profile which varies in the vertical direction from the top of the plate to the bottom thereof, whereby the thickness of the film at the bottom of the plate is greater than at the top. Since the local heat transfer coefficient between the film and the surface of the common plate member varies inversely with the condensing film thickness, the overall heat transfer from the condensing chambers to the boiling chambers becomes increasingly less from the top of the plate where the thickness of the film is relatively thin, to the bottom of the plate where the thickness of the film is relatively thick.
The boiler/condenser assembly of the present invention is an improvement over the boiler/condenser assembly disclosed in the above-mentioned U.S. Pat. No. 4,671,856 in that the flow path of the condensing film is shortened, and consequently its thickness is decreased to thereby improve the overall heat transfer between the boiling and condensing chambers.